FIG. 1 shows an example a radio that includes an amplifier chain. This exemplary radio includes baseband processing 110, a frequency upconverter 120 (or alternatively, a frequency downconverter), an amplifier chain 130, and an antenna 140. The frequency converter 120 receives a local oscillator that sets the carrier frequency of the amplifier chain 130.
Typically, radio frequency amplifiers utilize resonant stages for impedance matching purposes. Generally, the gain of a resonant stage is maximum at the stage's resonant frequency, and decreases at lower and at higher frequencies. FIG. 2 shows an example of a frequency response of an amplifier chain of a radio. In general, the radio signal band is not located exactly at the resonant frequency, where the frequency response is relatively flat, but rather at some frequency offset away from the resonant frequency, where the frequency response is relatively sloped. A possible location for the signal band 220 is shown in FIG. 2. As a result of the sloped frequency response, some frequency components of the signal obtain more amplification than others. Although the effect is ordinarily small for a single resonant stage, the combined effect of multiple resonant stages can be significant.
One solution to this problem is to adjust the resonant frequency of one or more of the resonant stages in the amplifier. This is unattractive, however, because (1) the technologies used for radio frequency amplifiers are in some cases not amenable to large scale integration, and (2) the parasitic capacitance associated with the tuning circuitry decreases the efficiency of the amplifier. In a direct conversion radio system, there is of course no possibility of performing any compensation for the asymmetric response at an intermediate frequency.
It is desirable to have an apparatus and method for compensating for the asymmetric response of a radio frequency amplifier.